1. Field of the Invention
This invention relates to spatial light modulator display systems, more particularly to addressing schemes for these systems.
2. Background of the Invention
Spatial light modulator display systems typically include a spatial light modulator comprised of an x-y array of individually controllable elements that are used to modulate picture elements (pixels) of an image. Examples of these modulators include Digital Micromirror Devices.TM. (DMD.TM.), Actuated Mirror Arrays.TM., liquid crystal cells, grating light valves, and plasma display panels. Some of these examples operated in an analog fashion, where the amount of light transferred to any pixel is determined by how far the corresponding cell moves, or how much light is allowed through that cell. Others operate digitally, where the cell either transfers light to the image or not.
The digital mode of operation raises unique problems since the human eye has an analog response. This analog response requires that the digital cells use a technique called pulse-width modulation (PWM); In PWM techniques, the display signal undergoes digital sampling, resulting in a predetermined number of samples, each having the same number of bits. These bits are then used to address the individual cell in for time periods proportional to the bits' significance (i.e., the most significant bit receives the most time to display its data). Systems with a higher number of bits per sample provide better images.
Addressing the cells typically involves transferring the data for a given bit to the activation circuitry for a cell, causing the cell to respond to that data, then illuminating the cell to modulate the light in the manner required by that bit of data. The tasks of storing, transferring, activating and illuminating the cells must be repeated several times in a relatively short time to achieve high quality images. A typical display system operates at 60 Hz, so each frame of data has only 1/60 of a second (16.7 milliseconds) in which to be displayed. In a sequential color system, where the modulator is illuminated with each of the three colors, red, green and blue, in sequence, each color receives one-third of that time (5.57 milliseconds).
In current spatial light modulator display systems, the largest number of bits achievable in a sequential color system is typically 8 bits. Eight bits of data must divide the 5.57 milliseconds up between them, with the most significant bit receiving approximately one-half that time (2.79 milliseconds), and the least significant bit receiving roughly one-two hundred fifty fifth of that time (20 microseconds). The cells must have a switching time fast enough to display the data for the least significant bit (LSB) in 20 microseconds to achieve 8-bits of resolution. Higher numbers of bits require even faster switching speeds.
For larger display systems, such as digital cinema, resolution higher than 8 bits is necessary to achieve film quality images with spatial light modulator displays. In some examples, such as the DMD.TM., 10 bits can be achieved. Therefore, for digital cinema quality images, a method is needed that will allow spatial light modulators to display more than 10 bits of resolution without requiring an increase in switching speed.